2G network parameters

1. GSM Network SDCCH Congestion &
Solutions

3. i
Contents
1 SD Seizure Signaling & Relevant Counters............................................................................................. 1
1.1 SD seizure and release flow.............................................................................................................. 1
1.2 Definition of SDCCH congestion indicator ...................................................................................... 2
2 Classification of SDCCH Congestion Causes .......................................................................................... 5
3 Procedure of Checking SD Congestion SD .............................................................................................. 7
4 Typical Cases of SD Congestion.............................................................................................................. 11
4.1 SD congestion due to LAPD delay caused by transmission fault ................................................... 11
4.2 SD congestion due to large quantity of LOC (caused by cross-LAC coverage)............................. 11
4.3 SD congestion due to strong interference ....................................................................................... 12
4.4 SD congestion due to large quantity of LOC caused by HLR cutover............................................ 14
4.5 SD congestion due to short message pager ..................................................................................... 16

5. 1 SD Seizure Signaling & Relevant
Counters
1.1 SD seizure and release flow
Fig 1-1 SD Seizure and Release Flow
As shown in Fig 1-1, during LOC and the early stage of MOC and MTC process, MS
usually seizes SDCCH to exchange signaling. The signaling flow is as follows:
· After receiving Channel Required message, BSC applies for SD from database,
and the counter for SD seizure attempt times increases by 1;
· When database allocates SD, the number of successful SD seizure increases by
1;
· BSC sends SD activation message to BTS, then the number of SDCCH
allocation attempts increases by 1;
· BTS responds to BSC with a response message of SD activation, BSC increases
the number of successful SDCCH allocation by 1;
· BSC sends Immediate Assignment message to MS through BTS, BSC increases
1

6. GSM Network SDCCH Congestion & Solutions
2
the number of SDCCH assignment attempts by 1;
· MS report SABM frame, BTS reports Est Ind message to BSC, and BSC
increases the number of successful SDCCH assignment by 1.
1.2 Definition of SDCCH congestion indicator
Table 1-1 SDCCH Congestion Indicator Definition
KPI name SDCCH blocking rate
Definition Number of signaling channel blocking times*100%/ Number of signaling channel
call attempts
V2 (2.97) (C11625-C11626+C11697)*100%/(C11625+C11696)
Counter
formula
V3 (6.20)
(C900060005+C900060011+C900060039)*100%/(C900060003+C9
00060010+C900060038)
SD congestion means that upon receiving Channel Required message, BSC applies for
SD from database, while BSC has no SD for allocation (all SD resource are blocked or
occupied).
When BSC fails in applying for SD, it sends a message Immediate Assignment Reject
to MS, commanding MS to request for access after a period of time (set on T3122); and
BSC increases the counter for SDCCH congestion by 1. Message flow of Immediate
Assignment Reject can not solve SD overflow problem, but can help relieve and
smoothen it.
Table 1-2 Timer T3122
Protection period of access attempt（T3122,s）
Value range 0 ~ 255
Unit s
Default value 10

7. Chapter 1 SD Seizure Signaling & Relevant Counters
3
Description
If network has no appropriate channel to allocate to MS after it has received
channel request message from MS, it sends Immediate Assignment Reject to
MS. In order to avoid radio channel congestion which may be caused by
continual channel request from MS, timer parameter T3122 (the so called
Wait Indication message) is included in Immediate Assignment Reject
message.
After it receives Immediate Assignment Reject message, MS can not start a
new call until T3122 expires. This timer is one of the system control
parameters, which is sent to MS in the Immediate Assignment Reject.
Recommended normal value for T3122 is 10~15s, and 15~25s in areas with
dense traffic.

9. 5
2 Classification of SDCCH Congestion
Causes
From the aspect of singling flow, there are two main types of causes of SD congestion.
Too many Channel Required exceeds network capacity and all SDCCHs are occupied.
· Too-many Channel Required means the cell is busy, while few SDCCH are
configured, which results in frequent occupancy of SDCCH and overflow.
· When checking SD congestion, what shall be considered is if there is possibility
of traffic increase.
Some operations at OMCR (such as HLR configuration or re-planning of LAC) may
lead to traffic increase in network.
Too long occupancy period of SDCCH due to non-in-time ending of signaling flow;
· If signaling flow doesn’t end in time, which means channel activation/release
period is too long due to some reason (say, transmission fault), it will lead to
long period of SDCCH occupancy and reduce SDCCH resource, and eventually
result in SDCCH overflow.
· Too-long channel activation/release period causes MS to repeat Channel
Request again and again (the number of repetition is decided by system
parameter MaxRetrans), and to occupy SDCCH repeatedly, which worsens
SDCCH overflow.

11. 7
3 Procedure of Checking SD Congestion
SD
1. Read performance report, check if all cells or some cells under the BTS are
congested;
It rarely happens that all cells under the BTS are congested. If it happens, the
causes are usually related to terrestrial transmission and parameter
configuration.
2. Read performance report, check if channel allocation failure (i.e. channel
activation expires or fails) happens during SD congestion;
Note:
Too large LAPD flow will lead to LAPD transmission delay, which causes timer
expiration before channel activation is completed. This kind of timer expiration shall
be differentiated from that caused by transmission fault.
3. Check radio access measurement, analyze the access reason of Channel Request
which causes SD congestion, count the number of Channel Request attempts and
success times due to different reasons, and compare indicators with those in
normal period..
The access cause of Channel Request falls into 4 types:
· MOC (Mobile-Originating Call);
· MTC (Mobile-Terminating Call)；
· LOC (Location Update)；
· Other reasons (call re-establishment)
Generally, the number of LOC attempts takes 35% of the total Channel Request times
The percentage may increases to 70%, if the site is located at LAC boundary.
Number of MOC Channel Request and that of MTC Channel Request are almost equal,
which are related to local subscribers’ call conditions;
Other reasons rarely happen in normal conditions.

12. GSM Network SDCCH Congestion & Solutions
8
Here are explanations for each type of cause:
1) If there are a big number of attempts due to other reasons, and all end in failure,
the cause can be confirmed to be interference.
2) When the number of MOC attempts is big, and even exceeds the number of
LOC, the reasons could be:
· There is MS malicious pager operating in the network (which is commonly seen
in Shenzhen and Quanzhou);
· Interference exists.
Judge the reasons through analyzing number of MOC success.
Although operation of MS malicious pager causes great number of MOC
attempts, number of MOC success increases accordingly;
Interference won’t increase the number of MOC success.
3) If number of LOC increases abnormally, check if there are any changes on
network parameters, such as re-planning of LAC or amendments of HLR and
VLR.
4) Generally speaking, SD congestion caused by LOC won’t bring large number of
assignment failures. If SD congestion is accompanied with large number of
assignment failures, it’s very possible that the cell traffic volume is high or
co-channel interference exists.
Note:
For BSCV2 system, basic measurement includes the number of attempts/success of
MOC, MTC, LOC and other accesses. For iBSC system, there is a special radio access
measurement, which needs to establish measurement task. In CS basic measurement,
number of MOC/MTC attempts and number of MOC/MTC success are included,
through which we can calculate the number of attempts/success of LOC access.
5) Make enquiries and find out if there are newly-setup sites, adjustments on
LAC/HLR;
6) Check the performance report of the week when the problem appears, analyze if
SD congestion exists for a long time during busy hours.
If SD congestion is a long standing issue, and there’s no big fluctuation in the

13. Chapter 3 Procedure of Checking SD Congestion SD
9
number of MOC, MTC, LOC attempt and success times, this means the cell is
busy and its traffic volume is high, and expansion is needed.

15. 4 Typical Cases of SD Congestion
4.1 SD congestion due to LAPD delay caused by transmission fault
The performance report showed that during SD congestion (SDCCH occupancy failure
counter), the number of SDCCH allocation failures was high.
The signaling flow shows that BTS didn’t respond to the Channel Activation sent by
BSC.
Fig 4-1 No BTS Response to Channel Activation
There was transmission warning during SD congestion;
After BTS was reset, the problem disappeared;
After transmission was adjusted, the problem was completely solved.
4.2 SD congestion due to large quantity of LOC (caused by
cross-LAC coverage)
Performance report of a site showed large amount of SD congestion in busy hours of
each day.
11

16. GSM Network SDCCH Congestion & Solutions
From analysis of radio access measurement, it’s discovered that the SD congestion was
caused by large quantity of LOC.
When checking the site map, engineers found the site was located between two
location areas (LAC), the cell belonged to LAC09523. A road, 1.5km from the site, was
under the coverage of LAC09533.
Through signaling tracing, we could see the large quantity of LOC under the cell was
from LAC09533 to LAC09523.
Fig 4-2 Signaling Showed Large Quantity of LOC under the Cell
After DT on site, it’s discovered that the road should be covered by LAC09533, but the
congested cell signal was very strong there, which means it stretched its coverage to
the road, and resulted in frequent LOC on the road and SD congestion.
4.3 SD congestion due to strong interference
On one night, large amount of SD congestion occurred at a BTS (2) in a town, which
lasted for a long time.
· Normal condition didn’t return even after reset of CMM and TRM;
· Congestion disappeared after adjustment of frequency and BCC;
12

17. Chapter 4 Typical Cases of SD Congestion
13
· Congestion phenomenon appeared again after frequency and BCC were changed
back;
· Congestion totally disappeared 30 minutes after adjustment on TA access
threshold.
Observed signaling, discovered the SD congestion was caused by large quantity of
abnormal Channel Request, and all the Imm Assign generated from these Channel
Request ended in failure.
All abnormal Channel Request appeared once every four frames; all RA was 01; TA
diminished from 63 to 0, then restarted from 63 after 815 frames; level value remained
63.
Fig 4-3 Case III-Large Quantity of Channel Request
Normal condition didn’t return even after the reset of CMM and TRM, which means
the problem was irrelevant to BTS hardware and software;
The problem disappeared after adjustment of frequency and BCC, but reoccurred when
the frequency was changed back, this means the problem involved in frequency;

18. GSM Network SDCCH Congestion & Solutions
14
Considering Channel Request rules, engineers confirmed there was an interference
signal whose frequency was the same as the site’s, and the signal just contained all the
training sequence of AB frame. The interference signal was periodical and it created
periodical deviation to timeslot 0 window. Just because of this deviation, TX changed
periodically. Besides, the interference signal just interfered timeslot0. Therefore,
adjustment on TA access threshold could only relieve the problem, but couldn’t solve it
completely.
There was army garrisoning in the area, the interference signal was probably sent from
the army.
4.4 SD congestion due to large quantity of LOC caused by HLR
cutover
Large amount of SD congestion occurred in a GSM network on Nov. 19th
, 20th
, 21st
and
22nd
, 0:30-1:00am.
Performance report showed there was a large number of LOC Request during
congestion.
The measurement data of LOC at MSC showed that at 0:30am on Nov.20th
, the number
of inter-MSC LOC attempts jumped from 2000 to over 20000, and it returned to
normal after half an hour;
Table 4-1 Case IV-Dramatic Increase in LOC Number

19. Chapter 4 Typical Cases of SD Congestion
At 0:00am and 0:30am on Nov. 21st
, the number of inter-MSC LOC attempts jumped
from 3000 to over 17000 twice.
Number of inter-MSC
LOC success-UMTS
Number of inter-MSC
LOC attempts-GSM
Number of inter-MSC
LOC success-GSM
1677024752008-11-20 1:30
1787025612008-11-20 1:15
2506032152008-11-20 1:00
6695076322008-11-20 0:45
187970201972008-11-20 0:30
1634024322008-11-20 0:15
1987027752008-11-20 0:00
1812025612008-11-19 23:45
2011027732008-11-19 23:30
2081029662008-11-19 23:15
Time
Table 4-2 Case IV- Dramatic Increase in LOC Number
Number of inter-MSC
LOC success-GSM
Number of inter-MSC
LOC attempts-GSM -GSM
Number of inter-MSC
LOC success-UMTSTime
2467
2532
4134
15562
4809
10138
7287
2657
2133
2311
0
0
0
0
0
0
0
0
0
0
3725
3790
5460
17118
6322
11784
8859
3954
3462
3674
2008-11-20 23:15
2008-11-20 23:30
2008-11-20 23:45
2008-11-21 0:00
2008-11-21 0:15
2008-11-21 0:30
2008-11-21 0:45
2008-11-21 1:00
2008-11-21 1:15
2008-11-21 1:30
At 1:00am on Nov. 22nd
, the number of inter-MSC LOC attempts jumped from 6000 to
over 16000, and it returned to normal on the early morning of 23rd
and 24th
.
On the early morning of 25th
, the number of LOC attempts was 15000.
These LOC attempts matched closely with the time point of SD congestion and the
15

20. GSM Network SDCCH Congestion & Solutions
16
cause of SD occupancy.
There was no obvious change on the number of inter-MSC LOC attempts during SD
congestion.
During congestion period, the number of TMSI re-allocation also increased
dramatically.
On-site engineers reported that Nokia was cutting over Nortel HLR when SD
congestion occurred. Therefore, we concluded that the cutover of HLR caused MS to
make large quantity of inter-MSC LOC at midnight, resulting in SD congestion. If it’s
BSS fault, then not only the number of inter-MSC LOC, but that of intra-MSC LOC
will be affected.
4.5 SD congestion due to short message pager
On Nov. 20th
, SD congestion of different degrees occurred in 6 cells under one BTS.
Performance report showed allocation failures of different levels in the cells.
Transmission was confirmed with no problem after investigation.
Checked performance data of radio access, discovered that the cause of large quantity
of radio access was MS sending short message:
For example, at 10:30, in cell3 the number of MOC Access Request reached 9959
within 15 minutes, while the number of successful access was 1070, and 1045 of the
1070 times were short message requests.
By observing other daily data of the BTS, engineers found the traffic volume was not
high.
It’s concluded that the problem was caused by heavy transmission load. Mass-sending
device of short message caused heavy transmission load, which led to LAPD delay, and
then led to SD allocation overtime, wasted SD resource, and finally resulted in SD
congestion.